It is known that cathodic protection is effective to prevent corrosion of reinforcing steel in concrete bridge decks, support structures, and parking garages, which are subject to extensive damage by corrosion of the steel reinforcement due to the presence of salt and moisture in the normally alkaline concrete environment. Such damage of reinforced concrete by corrosion results more particularly from the practice of spreading large amounts of salt on roads in winter, while coastal structures are attacked by seawater and salt spray.
Existing cathodic protection techniques for reinforcing steel are nevertheless limited by the anodes presently available for this purpose, which must have a long life and a chemically resistant anode structure that is readily adaptable from case to case according to the size and configuration of the reinforced concrete structure to be cathodically protected from corrosion.
A known type of polymer anode commercially availiable for use in impressed-current cathodic protection systems consists of a carbon loaded, current conducting polymer body with a copper core and operates at a current density limited to a maximum of about 0.02 A/m.sup.2 to avoid causing damage to the polymer anode surface.
Another type of anode which is used for cathodic protection of reinforcing steel consists of carbon fibers which are placed in a groove in the concrete, the groove then being filled with a grout of electronically conductive carbon-loaded backfill. Here again, the use of carbon presents serious limitations, since this material is subject to high operating voltages and therefore a limited lifetime as an anode. This is a serious limitation since replacement of anodes embedded in concrete is very difficult. This type of anode also has a high electronic resistivity, so that current can be carried longitudinally only over very short distances through the carbon fibers.
Other anodes which are traditionally used for impressed current cathodic protection are constructed of platinized titanium or platinized tantalum with a more electronically conductive copper core. Such electrodes are often used for cathodic protection of underground pipelines, well casings, ship hulls, jetties, drilling rigs, and oil platforms. These electrodes are expensive and must therfore be used at a higher current density, up to 1000 A/m.sup.2 in some cases. The expense of such platinized titanium or tantalum electrodes entails special design problems since a very low current density must be applied to the structure being cathodically protected. This results in a mismatch of current density between anode and cathode. Various system designs attempt to accomodate this mismatch, usually by installation of small anodes at certain locations which are intended to protect large structures over great distances. Unfortunately, this often leads to unforeseen current density disparities and inadequate protection of more distant parts of the structure.
It is understood that continuous, wire-like flexible anodes for example are more suitable for cathodic protection of many structures such as underground pipelines in particular but until now such anodes were not capable of functioning over extended periods of time.
Numerous composite electrodes have moreover been proposed which comprise a polymer material combined with a dispersed conductive filler, or an electrocatalyst, or both. The state of the art relating to such composite electrodes may be illustrated for example by U.S. Pat. No. 3,629,007; 3,751,301; 4,118,294; 4,473,450 and European Patent Application No. 0 067 679.